1. Field of the Invention
The present invention relates generally to the field of well fracturing. More specifically, the present invention discloses a propellant assembly for fracturing wells.
2. Statement of the Problem
Propellant charges have been used for many years to create fractures in oil, gas and water formations surrounding a well. FIG. 1 is a cross-section diagram of a well 10 with a packer 12 and a series of propellant charges 20. The propellant charges 20 are ignited to rapidly generate combustion gases that create sufficient pressure within the well bore to generate fractures in the surrounding strata.
In order to achieve proper pressure loading rates and adequate minimum pressures for sustained periods of time sufficient to extend fractures in the surrounding formations using gas-generating propellants, it is necessary that a sufficient surface area of propellant be burning to generate the volume of gas required to extend such fractures, as gas generation is a function of the surface area of the propellant burning at any given time. If ignition of the propellant is limited to small areas of the outer surface of the propellant, then the flame from the initial burning area of the propellant must spread across the face of the propellant to ignite the remaining surface area. This flame spread rate is a key limiting factor to achieving proper pressure loading rates and adequate minimum pressures for fracture propagation in the surrounding formations. If the flame spread from a localized ignition point is too slow, then the burning surface area at any given point in time will be limited, and the overall time that the propellant burns to completion may have to be extended sufficiently to compensate for the reduced amount of time that pressures exceed the minimum required fracture extension pressure, resulting in a longer but less efficient propellant burn.
In addition, the propellant burn should be predictable and reproducible for the purpose of accurately modeling the fracturing process. It is difficult to accurately model a propellant burn unless the entire exposed surface of the propellant is ignited almost simultaneously. Modeling of propellants has been contemplated in the past, but with the assumption that ignition of the propellant surface over the entire exposed area of the propellant is simultaneous. Practically speaking, such simultaneous ignition is difficult to achieve.
The problem is further complicated by the following. When propellants are submerged in well fluids such as water (or water and KCI), flame spread rates tend to decrease. In addition, certain chemical coverings that are used as surface coatings on propellants to prevent leaching of the propellant fuel oxidizers into the surrounding well fluids also tend to inhibit the flame spread rate, thus exacerbating the problem. When such coatings are not applied to the surface of the propellant, sufficient leaching of the fuel oxidizer takes place over relatively short periods of time (i.e., 1 hour) to result not only in a reduction in the available energy to do work on the formation, but further, creation of an outer boundary layer absent of fuel oxidizer and comprised primarily of the propellant binder, which tends to inhibit the flame spread rate because the exposed fuel oxidizer in the binder has been leached away. Furthermore, because gas generation is a function of the area of propellant burning at any given time, it is also useful to engineer a propellant fracturing system that accounts for the required initial burning surface area to provide adequate pressure rise, in addition to taking into account the flame spread rate.
In summary, the problem consists of igniting sufficient surface area of propellant simultaneously to overcome the effects of not only a limited flame spread rate, but also to mitigate the effects of any sealing coating placed on the propellant. In addition, one must be able to accurately predict the amount of gas generation by burning of the exposed surface area at any given point in time for proper modeling.
3. Solution to the Problem
The solution to the problem is to rapidly ignite the entire outer surface of the propellant charge by wrapping the ignition cord around the propellant charge in order to produce a burn that is reproducible, and can be accurately modeled to predict the resulting conditions in the well and surrounding strata during the fracturing process.